It is known to fabricate rivets from a wrought aluminum alloy containing 0.20 to 0.80% Si, 0 to 0.70% Fe, 3.5 to 4.5% Cu, 0.40 to 1.0% Mn, 0.40 to 1.0% Mg, 0 to 0.10% Cr, 0 to 0.25% Zn, 0 to 0.20% Ti +Zr, other elements in an amount of 0 to 0.05% each and in a total of 0 to 0.15%, balance aluminum (percents by weight).
This alloy has the Material No. 3.1324 in accordance with DIN Standards (German Federal Republic Standards) and the Number 2017 of the Aluminum association. It is mainly used to make rivets in the aircraft industry in accordance with LN 9197, 9198 and 9199. As supplied, the alloy has an ultimate tensile strength of 215 to 295 N/mm.sup.2 and transversely to the longitudinal axis has a shear strength of 255 N/mm.sup.2.
In accordance with page 4 of Annex 1 of Werkstoff-Legierungsblatt WL 3.1324, Issue June 1983, rivets made of this alloy must be solution-heated and quenched immediately before they are deformed. The rivets must be in a so-called unstable state while they are closed and the closing must be completed within two hours after the quenching. If the rivets cannot be closed within two hours after their quenching, they must be stored in freezers at -17.degree. C. But even in that case the rivets must be closed within one week. Rivets which have been precipitation-hardened at room temperature or have been stored for an excessively long time can be heat-treated again about five times.
No time limits need to be observed when rivets are closed which have been precipitation hardened at room temperature But in that case a formation of cracks in the closing head and a reduced fatigue limit of the riveted joint must be expected.
The use of the rivets is rendered very difficult by the regulations which permit a closing of the rivets only when they have been soft-annealed and subsequently quenched. In the manufacture of new aircraft and other new riveted articles, the processing requirements can be met by the use of suitable equipment and a suitable organization. But even in that case, errors and confusion cannot be precluded. The regulations for the closing of the rivets are particularly undesirable in repair work, which may be required on any airport, where the equipment and personnel required for a closing of the rivets in accordance with the regulations may not always be available.
For this reason there have been numerous attempts to replace the above-mentioned alloy by a material which retains a high ductility for a prolonged time so that the rivets can be closed without disadvantages at any time after they have been annealed and quenched.
The restrictive conditions need not be met if Alloy WL 3.1324 is deformed after it has been precipitation-hardened at room temperature. But in that case a formation of cracks will be more likely and the riveted joint will have a lower fatigue limit than when the rivets are deformed in the recommended state (see page 4 of Annex to WL 3.1324).
In accordance with another proposal, alloy AlZnMg AA 7050 is recommended as a material for rivets which retain a high ductility for a long time. Said rivets are precipitation-hardened at elevated temperatures in two stages and the temperatures at which they are precipitation-hardened must be very carefully controlled if the intended strength properties are to be achieved. Besides, said material is liable to form stress cracks and is rather expensive.
It is also desired to use a known alloy that has already been used for this purpose in the production of rivets which retain a high ductility for a long time because a new alloy could not be used unless it meets the requirements also in all other respects and this would require extensive technical tests and prolonged licensing procedures.